Several schemes are currently used to transport data from one point to another in data communication networks. The telecommunications industry has long used circuit switched, isochronous schemes to transport real-time voice information between points in a network. Circuit switched transmission reserves a guaranteed bandwidth to all users who are granted access to the network. Moreover, this bandwidth is uniformly distributed in time and is configured so that a minimum amount of delay occurs in transporting the information from point to point. For example, a typical voice channel transports 64 kilo-bits per second (Kbps) with only a few milliseconds round trip delay. The typical voice channel uniformly transports one byte or octet of data at regular 125 microsecond intervals.
Computers and other nonreal-time data generating devices may use such circuit switched networks. However, such "computer data" does not typically benefit from the rigid uniform bandwidth requirements of circuit switched isochronous transmission. Rather, the transmission of computer data is typically impaired by the relatively low bandwidth constraints which characterize a single, circuit switched channel. Moreover, circuit switched service reserves bandwidth in the network even though the user for whom such bandwidth has been reserved is not utilizing the bandwidth. Consequently, computers and other nonreal-time data generating devices may not be able to effectively utilize the reserved bandwidth over a long period of time, and the use of circuit switched service for computer data often proves expensive. Therefore, packet switched networks have evolved to address needs associated with the transmission of computer data.
In particular, forms of computer networking known as Local Area Networks (LANs) have emerged recently. LANs can be categorized in two major categories. The two categories primarily differ from one another in the manner in which they grant access to a shared medium. One category provides a deterministic access service, and the other category provides a random access service. Neither category provides a guarantee of uniform average bandwidth to its users. Consequently, such networks are generally unacceptable for transporting real-time demanding information. However, both categories typically allocate the full available bandwidth of the media transporting the data to a user who has been granted access to the network. Thus, such packet switching schemes favor the transmission of computer data when compared to circuit switched schemes because they do not reserve unused bandwidth. Moreover, overall bandwidth utilization improves, and such packet switched services may be provided to computer users more cheaply than a circuit switched service.
A token passing technique is commonly used to provide a deterministic access service in ring-shaped LANs. In such a "token passing ring", nodes in the ring may transmit data only when they capture a token, regardless of whether the media over which data is being transmitted is currently idle. Typically, there is only one token in the ring at any given time. Therefore, only one node has authorization to transmit, and that one node is the node that holds the token at any given instant in time. Regardless of whether the node holding the token has some information to transmit, the token-holding node is required to release the token after some maximum specified period of time. Upon release of the token, the token reaches the next node downstream, and the process repeats. In general, utilization of the data transporting medium may be relatively high when a network is configured as a token passing ring. However, undesirably long access times characterize conventional token passing rings because a node wishing to transmit data must wait for the token to circulate to it through the ring before it can initiate a transmission. Consequently, for either real-time data, large networks, or small networks when data traffic load is light, such access delays are particularly undesirable.
A carrier sensing multiple access/collision detection (CSMA/CD) service represents a conventional random access technique which is commonly associated with bus-shaped LANs. In CSMA/CD, if the medium over which data to be transmitted is not currently being used to transmit data, each node coupled to the medium may assume that the media is immediately available. Therefore, any station is permitted to begin a transmission after detecting the absence of a carrier prior to the transmission. If two or more stations happen to begin transmissions within a short interval of time, their transmissions collide and neither transmission successfully reaches its destination. However, in CSMA/CD the nodes are capable of listening to their own transmissions and of detecting such a collision. The nodes then stop transmitting for an arbitrary period of time, after which they try again.
Performance characteristics of CSMA/CD differ from those of token passing rings. Overall bandwidth utilization is typically less with CSMA/CD. However, when the network carries only a low data traffic load as compared to the total bandwidth of the medium, the access time improves over token passing rings. Still, access delay and throughput is not guaranteed. Consequently, for real-time data applications or for networks experiencing heavy data traffic, CSMA/CD services demonstrate undesirable performance characteristics.
In most applications, a network's data traffic load is not constantly high or low but changes dynamically according to the time of day. Therefore, regardless of whether a network provides a token passing ring service or a CSMA/CD service, it may be expected to provide inadequate service in one way or another at various times of the day.
Moreover, such conventional token passing and CSMA/CD LANs do not adequately serve the needs of a metropolitan area network (MAN). A MAN typically occupies a much larger geographical area than a LAN, supports more stations than a LAN, and permits indirect attachment of stations. In both conventional token passing and conventional CSMA/CD LANs, a network may transport only one data item at any instant in time. In other words, multiple, simultaneous accesses by various stations are not permitted. Although such limitation may not greatly impact a smaller LAN, a larger MAN would be significantly harmed by such single access schemes due to the increased propagation delay associated with transmitting data over large distances and the potentially large number of stations attached to the MAN.
Still further, such conventional token passing and conventional CSMA/CD access techniques do not adequately adapt to a network where each node in the network supports a multiplicity of users. Such multiplicity of users may include additional LANs or MANs which couple through a bridge. Conventional token passing and CSMA/CD LANs utilize a set of software procedures called Media Access Control (MAC) to format and transmit messages which the network carries. A message typically includes a relatively large address field, typically around 48 bits. The address field is used to specify message recipients. When each node in the network supports a multiplicity of users, the MAC sublayer in each node must open the MAC level message, examine the address field, and perform a sort operation to determine if one of the users supported by the node is the specified message recipient. This is a cumbersome process, and it must be completed before a node is able to dispose of the message. All nodes repeat this process. Consequently, overall performance suffers when conventional LAN switching and access techniques are utilized in large networks.
As discussed above, a conventional circuit switched service supports the transmission of real-time data, but a conventional packet switched service fails to adequately transmit real-time data. Conversely, a conventional packet switched service efficiently transmits nonreal-time "computer data", but a conventional circuit switched service fails to adequately serve non-real-time data needs.
In order to address problems related to the incompatibilities between circuit switched service and packet switched service, a hybrid scheme referred to as a slotted ring (or bus) has been developed. In a slotted ring the entire capacity, or bandwidth, of the network medium is time divided into time slots of equal size. The slotted ring typically allocates a number of slots to a circuit switched service and the remainder of the bandwidth to a packet switched service. This allocation may be done dynamically in some cases. The circuit switched portion of the bandwidth provides a uniform isochronous channel which is suitable for the transmission of real-time demanding data. Although the slotted ring permits transmission of both real-time and computer data, it does not improve upon the performance of LANs, as discussed above. Moreover, it provides an "all or nothing" approach to data transmission services where an instantaneous access time is provided by the circuit switched service and a poor access time is provided by the packet switched service.
Consequently, a need exists for an intermediate class or classes of data transmission services with access performance levels between a circuit switched, isochronous service and a packet switched service. Such intermediate services would be better adapted to the needs of high speed data users, real-time data users, such as video users, and computer users who require relatively quick access times or a guarantee of a minimum available bandwidth. Moreover, a need exists for a network which improves bandwidth utilization when distributed over a metropolitan area and which efficiently supports a large number of stations.